In the past, subsurface safety valves ("SSV's") have been controlled from hydraulic control systems from the surface. Hydraulic control systems are commonly used on production rigs for control of surface safety components. The SSV is located at or adjacent the base of the wellbore, or in a location immediately above the producing zone at the time. In emergency situations, a rapid shutdown of the SSV is required. The SSV's of prior designs have been actuated by movable sleeves, which have in turn been actuated by a hydraulic system from the surface. In applications involving great depths, the auxiliary tubing, run adjacent the production string for control of the SSV, develops considerable hydrostatic head pressures at the control mechanism downhole adjacent the SSV. To compensate for the developed static head pressures from the control fluid column in the control tubing, springs or other compensating devices have been used to counteract such forces. In these designs, the SSV remains closed until additional pressure is developed in the control tubing from the surface to overcome the spring force, thereby directing the control tubing pressure to shift the sleeve in order to open the valve. These systems were set to be failsafe because upon withdrawal or loss of the control system pressure, the spring acting on the piston would result in movement of the piston, with the final resulting action being the shifting of the sleeve, allowing the SSV to close.
Typical of such designs is U.S. Pat. No. 4,173,256. In that design, a spring biases a piston against the hydrostatic head in the tubing control line from the surface. Once the pressure is raised beyond the resistance of the spring and hydrostatic pressure from the annulus, the piston is displaced, compressing the spring and control pressure is communicated to the sliding sleeve to open the SSV. The SSV sleeve is spring-biased against production tubing pressure so that it retracts upon removal of control pressure, allowing the SSV to slam shut. Once the control pressure is removed, the spring in the control system pilot valve pushes the piston to close off the control fluid supply line and to vent the accumulated fluid adjacent the shifting sleeve behind the pilot valve piston into an area in fluid communication with the annulus.
One of the problems of the prior designs, particularly for applications involving significant well depths, was that high operating pressures were required for the control system in order to initiate movement of the sliding sleeve in the production tubing, as well as the pilot valve piston in the control system, for actuating of the SSV. The pilot piston spring had to resist higher hydrostatic heads in the control line due to the greater depth. Typically in these deep-well applications, the hydraulic control system used for other surface emergency components, would be of an insufficient pressure rating for the pressures typically required in a control system for an SSV which may be mounted 8,000-15,000 ft below the surface. Accordingly, operators would have to use discrete hydraulic control systems rated for the desired operating pressures for the sole purpose of actuation of the SSV. This involved additional expense to the rig operator. It also created space problems on the rig where space for operational components is at a premium. The hydraulic control systems used for surface components generally operated in the pressure range of between 1,000-3,000 psi. The pressure requirements for the SSV at deep installations could be as high as 10,000-15,000 psi. The higher pressure system required pipe and fittings rated for the higher pressure service and precluded the use of the standard hydraulic control systems normally present in a rig.
The apparatus and method of the present invention presents a configuration where the hydrostatic forces from applications at large depths have become inconsequential due to a balanced design for the actuation system. The actuation system is exposed to production tubing pressure on opposing surface areas of approximately equal area, thus putting the actuation mechanism in a force balance until the balance is upset by application of control pressure from the surface, triggering movement of the SSV. In another feature of the invention, the need for occasional purging of control fluid from the control system of an SSV is accomplished. Purging is particularly beneficial because uses of water-based control line fluids have increased sensitivities to contamination and breakdown. Traditional systems for control of SSV's from the surface involve systems that have a fixed volume, as opposed to one where the control fluid is circulated. A circulating system would require a pair of control lines down to the SSV and would increase complications in installation and operation. Without the ability to do purging or circulation, the control fluid could prematurely fail and damage control system components such as seals. In another feature of the apparatus and method of the present invention, a shuttle valve has been designed which facilitates the operation of the control system and, for each cycle of opening and closing the SSV, purges a fixed amount of control fluid from the system so that premature failure of system components such as seals does not occur.